Internally today we've discovered a breaking issues in 3.2 (Stable) Xamarin.Mac which prevents bindings projects from working correctly:
https://bugzilla.xamarin.com/show_bug.cgi?id=55401
This is fixed in 15.2 (Xamarin.Mac 3.4) which is in Alpha right now.
Until 15.2 lands in stable, users needing binding projects can either switch to the alpha channel (https://developer.xamarin.com/recipes/cross-platform/ide/change_updates_channel/#Change_the_Updates_Channel_in_Xamarin_Studio_and_Visual_Studio_for_Mac)
or revert back to Xamarin.Mac 3.0:
https://dl.xamarin.com/XamarinforMac/Mac/xamarin.mac-3.0.0.398.pkg